The present invention relates to the art of medical diagnostic imaging. It finds particular application in conjunction with spiral volume imaging with CT scanners and will be described with particular reference thereto. However, it is to be appreciated that the invention will also find application in conjunction with other types of volume imaging, with multiple single slice images, continuous rotating x-ray source images, gated imaging, and the like.
In spiral or helical scanning, the x-ray source or tube rotates continuously as the patient support table moves at a constant, linear velocity. In this manner, the collected data effectively represents a helical path of constant pitch through the patient. Conventionally, the data is stored and handled as a series of parallel planes, transverse to the longitudinal axis of the patient, or more specifically, as a three dimensional rectangular matrix of memory cells. See for example U.S. Pat. No. 3,432,657 to Slavin.
In order to fit the spiral collected data into a conventional three dimensional rectangular matrix, a series of parallel planes are defined through the spiral collected data, with a one plane per spiral revolution, e.g. at each 0.degree. of source rotation. During the data collection period, a series of views or fans of data are collected at preselected angular increments around the patient. Potentially, one view per plane, by convention the 0.degree. or 12 o'clock view, falls squarely in the plane requiring no averaging or weighting. For each remaining view of the plane, there is a pair of corresponding views or data fans, one from the revolution preceding the plane and the other from the revolution following the plane. These views are averaged or weighted in accordance with their relative distance from the plane. In this manner, a full set of weighted views is created to perform a conventional 360.degree. CT reconstruction algorithm. See U.S. Pat. No. 4,630,202 issued December 1986 to Mori and U.S. Pat. No. 4,789,929 issued December 1988 to Nishimura, et al.
One of the problems with the spiral scanning techniques is that excessive partial volume artifacts were caused in certain applications. Another problem is that the linear interpolation or weighting is only applicable to 360.degree. based revolution reconstruction techniques, not 180.degree. plus fan reconstruction algorithms. See U.S. Pat. No. 4,293,912 issued October 1981 to Walters.
Another problem with the linear interpolation technique is that it introduces errors in fourth generation scanners using source fan reconstruction. In a third generation scanner in which the x-ray source and an arc of detectors rotate together about the slice, each data fan or view is collected instantaneously in a plane parallel to the artificially defined transverse slices. In a fourth generation scanner, there is a parallel ring of stationary detectors surrounding the patient. With source fan reconstruction, each detector is sampled at monitored, time displaced intervals generating a view or fan of data as the source rotates behind the examination region. Because the patient moves longitudinally between the first and last data sampling of the view or data fan, the views are warped or canted along the spiral path. The linear interpolation scheme which assumes that the views lie parallel to the artificially defined planes introduces errors.
Another problem with the linear interpolation technique is that it is particularly sensitive to variations in the x-ray rotation speed, the velocity with which the patient is moved, and fluctuations in the output of the x-ray tube.
Continuous rotation of the x-ray source with a stationary patient has been utilized for gated scanning. See for example U.S. Pat. No. 4,868,748 issued September 1989 to Mori. In this technique, the patient remains stationary and the x-ray tube continuous to rotate in the same plane of the patient. In response to the R-wave of the patient's cardiac cycle, the x-ray tube or its shutter is gated on to collect a view of data. In this manner, data is collected over a plurality of cardiac cycles for constructing a stop-action slice through the patient's heart. Of course, this technique is not amenable to spiral volume imaging.
Another prior art CT scanner imaging technique includes the generation of two reconstructed images through the same slice but with different energies. The two different energy -images could be collected concurrently by pulsing the x-ray tube alternately at high and low energy levels to collect the high and low energy views alternately. See "Generalized Image Combinations in Dual KVP Digital Radiography", Lehmann, et al., Med. Phys. 8(5), Sept./Oct. 1981. Alternately, the two images could be collected sequentially, i.e. all of the views of the low energy image followed by all of the views of the high energy image. See "Evaluation of Prototype Dual-Energy Computed Tomographic Apparatus, I Phantom Studies", Calendar, et al., Med. Phys. 13(3) May/June 1986 and "Evaluation of Prototype Dual Energy Computed Tomographic Apparatus II Determination of Vertebral Bone Mineral Content", Vetter, Med. Phys. 13(3) May/June 1986. However, each of these techniques required two rotations per slice, one rotation to collect the low energy image data and one rotation to collect the high energy image data or one rotation to collect half the low energy and half the high energy image data and a second rotation to collect the other half of the low energy and the other half of the high energy image data. Because two revolutions per image set are required, the prior art dual energy imaging techniques are not suited to linear weighted helical scanning.
In accordance with the present invention, a new and improved imaging technique is provided.